1. Field of Invention
This invention is directed to a circuit that can be both non-permanently programmed and permanently programmed.
2. Description of Related Art
Fuses and fusible links are circuit elements that open by burning out or breaking when a relatively high current is applied. By selectively breaking or leaving intact specific fusible links, a circuit can be customized or programmed using these fusible elements. An antifuse is the opposite of a regular fuse. That is, an antifuse is normally an open circuit until a programming current is forced through it. Fuses and antifuses may be used to address many problems, including calibration requirements of analog circuits such as digital/analog converters, or current or voltage sources, logic synthesis circuits such as digital delays lines, or chip specific performance data to be used by the end system in which the chip will be used. These fusible and antifusible circuits are generally "programmed" after chip fabrication has been completed and during the wafer testing phase of chip production. The programming may be used to add additional resistors into a circuit to compensate for variations introduced during the manufacturing process or to compensate for oscillator frequency variations induced by manufacturing stress.
In many of these cases, it is desirable to simulate a programmed state before actually programming the device. For example, analog circuit calibration may require additional steps of simulation and refinement based on the previewed or simulated results obtained. These results are incorporated into further simulations to correctly calibrate the circuit during the testing phase. After testing, the circuit may then be permanently programmed.
Conventional programmed circuits employing, for example, fuses and anti-fuses, generally do not permit preview or simulation of the programmed circuit. Conventional programmed circuits require specialized packaging to ensure no overlay of the fusible or antifusible link occurs. That is, a conventional circuit can only be permanently programmed and not previewed. Once the conventional circuit is programmed, no further changes or refinements to the circuit are possible.
U.S. Pat. No. 6,037,871 to Watrobski et al. describes such a fusible link circuit including a preview feature that uses fusible links in combination with transistors to permanently set the value of an output. However, this fusible link circuit also requires special manufacturing and packaging techniques, as discussed above. For previewable devices such as those described in Watrobski, the device imposes specialized packaging, manufacturing, handling and cost limitations. Conventional fuse programming methods require that the selected device packaging technique be suitable for the programming structures. For example, the device packaging techniques need to facilitate air access to the fuse for burning and to avoid overlays that may act as heat sinks. A heat sink would increase the fuse blow temperature, which could possibly exceed the circuit temperature tolerance. Thus, manufacturers of products requiring these features must select packaging and fabrication techniques that are tailored to these fusible circuits and which are typically more expensive than non-programmable circuit fabrication and packaging techniques.
Erasable programmable read only memory (EPROM) and electrically erasable programmable read only memory, (EEPROM), devices may be programmed and reprogrammed. However, EPROM devices rely on specialized fabrication techniques and typically include a quartz window through which ultraviolet light of a specific wavelength may be introduced for several minutes to erase the chip in preparation for re-programming. In use, the quartz window is covered to prevent accidental erasure of the device. The EPROM devices require physical removal of the chip and or physical manipulation of the cover over the quartz window as well as considerable time to effect erasure in preparation for re-programming. Thus, EPROM devices require both specialized fabrication techniques and specialized handling during programming.
EEPROM circuits typically use floating gates surrounded by a much thinner insulating layer which can be erased by applying a voltage of the opposite polarity to the charging voltage to the non-floating gate. EPROM circuits overcome some of the EPROM device limitations with respect to the use of ultraviolet light to effect erasure. However, EEPROM devices also require special fabrication techniques in their manufacture. Furthermore, EEPROM devices require that special "opposite polarity" voltage levels be adopted for reading and writing to the EEPROM device.